It was once the emblem of US military success in Iraq - the city where American forces defeated their adversaries twice. First they drove out the army of Saddam Hussein, and later the Islamist militants who rose up against the occupation.

Less than a decade later, the two US foes have joined forces to seize back Tal Afar, and the machine guns and armoured vehicles that American troops left behind to defend its new-found freedom are being used by the very insurgents they sought to defeat.

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The arrival of the jihadists from the Islamic State of Iraq and al-Sham in northern Iraq has brought on sectarian insurgency anew, and loyalists to the late Saddam Hussein have once again picked up their arms to fight.

23.40 Iraqi forces appear to be rallying and bolstering their defence of Baghdad in the face of the Sunni extremists - that's according to the Pentagonat least.

We also have reason to believe - certainly indications - that the Iraqi security forces are stiffening their resistance and their defense and are coalescing, particularly in and around Baghdad, and that's encouraging," spokesman Rear Admiral John Kirby said today.

With Iraqi troops now receiving help from Shiite volunteers, Kirby said "it certainly appears as if they have the will to defend the capital."

22.50 Washington remains open to more talks with Iran over the crisis in Iraq, but is far from following in Britain’s footsteps to renew diplomatic ties, a US official said today.

Deputy Secretary of State Bill Burns had “met briefly” with Iranian officials on Monday in Vienna on the sidelines of nuclear talks with global powers, State Department spokeswoman Jen Psaki confirmed.

They discussed the need to support inclusivity in Iraq and the need to refrain from pressing a sectarian agenda,” she said.

“We’re open to continuing our engagement with the Iranians, just as we are engaging with other regional players on the threat posed by ISIL in Iraq.”

21.55 Iraqi Shi’ite and Sunni political leaders have made a joint call for national unity after a closed-door meeting was called in the wake of the recent sectarian violence.

The leaders, including Prime Minister Nuri al-Maliki, a Shi’ite, and Sunni Usama al-Nujaifi, the last speaker of parliament which dissolved this month, stood apart and listened as Maliki’s predecessor Ibrahim al-Jafaari called for “defending the state and protecting its sovereignty and dignity.”

They also called for avoiding sectarian grievances and forbade non-state actors from carrying weapons.

20.27 US congressional leaders are to meet President Barack Obama tomorrow in the White House to discuss the deteriorating security situation in Iraq, officials have said.

Democratic and Republican officials said that John Boehner, the House speaker, Nancy Pelosi, the House Democratic leader, Harry Reid, the Senate Majority leader and Senator Mitch McConnell, the Republican leader - will sit down with the president at 3 pm (8pm UK).

The meeting is "a part of (Obama's) ongoing consultations with congressional leadership on foreign policy issues, including the situation in Iraq," a White House official said.

Mr McConnell "asked the president to provide us with a strategy and a plan and it's his hope that those will be provided at the meeting," a McConnell aide told AFP.

19.25 Watch David Cameron's remarks on Isis, which he warned earlier today posed the "biggest threat" to British national security:

19.00 The Telegraph's Damien McElroy reports on the use of social media by Isis to advance its aims - you can read the full article here.

He writes:

Analysts believe that Isis’s savvy use of internet tools has help spread its reputation and drive recruitment. Stratfor, the US think tank, said that Isis used the hashtag #SykesPicotOver during the advance - referring to the 1916 deal between British and French officials to carve up the Middle East into respective zones of influence.

"Isis is making effective use of startling images depicting their operations, notably including mass executions of Iraqi soldiers in Tikrit last week. Their fearsome reputation, bolstered by such images, has translated into success on the battlefield, with Iraqi security forces fleeing strategic towns and cities rather than engage the militants directly," said Jordan Perry, an analyst at risk advisory company Maplecroft. "It is very likely that intelligence agencies in Europe, the US and elsewhere are monitoring the online chatter of Islamist groups – including Isis – very closely indeed."

18.10 Amidwidespread reports of violence perpetrated by Sunni Islamist Isis and its allies, primarily directed at the Shia population, signs of reprisals against Sunnis have begun to emerge.

Police said pro-government Shia militia killed nearly four dozen detainees after insurgents tried to storm a jail in Baqouba, northeast of Baghdad. Earlier, The Telegraph spoke to a resident of Tal Afar, who said two of his cousins had been killed in the massacre but claimed prison security guards were responsible.

A local morgue official told AP that many of the detainees had bullet wounds to the head and chest.

In Baghdad the bullet-riddled bodies of four men in their late 20s or early 30s, believed to be Sunnis, were found at different locations in the Shia neighborhood of Benouk, according to police and morgue officials.

17.00 The Telegraph's Ruth Sherlock is in Iraq, where she has been speaking to refugees from Tal Afar, the northern city which was seized by Isis yesterday.

One resident said that two of his cousins had been killed in the prison massacre. He works in military intelligence, but refused to give his name out of fear of reprisal.

My cousins are from Borgholieh village in the Tal Afar area. They and four others from this village were in the prison.

My two cousins were shot dead and I am told their bodies have been burned.

The prison is run by the Iraqi Counter Terrorism Unit - the special forces.

Suddenly the prisoners were told they would be free to go and they opened the doors: the security guards wanted to show that the prisoners had "fled".

Once they were running out of the prison they shot them in their backs. They used a PKC to gun them down.

All the dead are Sunnis. I heard they tried to burn their bodies after. A friend of mine gave me this information. He was there also. They shot him in the stomach but he managed to get away. He is now in Mosul with his family.

Isis supporters wave al-Qaeda flags in Mosul (AP)

16.50 British citizens and other nationals fighting alongside Islamist insurgents in Iraq and Syria pose the biggest threat to Britain's national security that exists today, David Cameron has said.

"No-one should be in any doubt that what we see in Syria and now in Iraq in terms of Isis is the most serious threat to Britain's security that there is today," Cameron told a joint news conference with Chinese premier Li Keqiang.

"The number of foreign fighters in that area, the number of foreign fighters including those from the UK who could try to return to the UK is a real threat to our country," he said.

Stefan Rousseau/ PA

16.42 Iraqi Shia volunteers who went to fight for Assad's regime against the mainly Sunni rebels are returning in droves to their homeland to join the battle against Isis, the Syrian Observatory for Human Rights has said.

Many had volunteered to defend the Sayyida Zeinab mosque, a revered Shiite shrine in southeast Damascus, from rebels based in the outskirts of the Syrian capital. But in the wake of the Isis offensive the volunteers have begun heading home in response to a rallying cry by top Shiite cleric Grand Ayatollah Ali al-Sistani, according to the monitoring group.

Iraqi volunteers headed home from the Mleiha area, southeast of Damascus, it said.

Their positions were taken over by fighters of Lebanese Shiite militant group Hezbollah, which has also intervened heavily alongside Assad's forces in Syria's civil war, Observatory director Rami Abdel Rahman said.

"The Iraqi pro-Assad fighters' pullout from the Mleiha area was accompanied by a relative lull in the fighting around there," he told AFP.

"But that does not mean the regime has been left defenceless, as Hezbollah has deployed new troops to fill the gap."

Iraqi men flash victory signs as they leave the main recruiting centre to join the Iraqi army in Baghdad (AP)

16.19 Police in Iraq said a roadside bomb exploded in central Baghdad on Tuesday, killing at least three people and wounding nine.

The blast took out a chunk of the pavement at the side of the busy road. Battered remains of market goods are strewn around.

Isis jihadists have pledged to seize the capital city, though they were repelled overnight in their attempt to take Baquba - at just 37 miles northeast of Baghdad, the closest they have so far come.

16.06 As the Isis offensive on Baghdad rages on, take a look at our updated Iraq crisis map (click to enlarge) to see where the latest frontline and fighting is taking place:

15.28 The contrast in the use of Twitter between the West's traditional foe Iran - whose president was pictured watching his country's World Cup opener in a tracksuit - and the Isis jihadists could not be more profound, writes Robert Tait, our Middle East Correspondent.

Hassan Rouhani cuts a benign, non-threatening figure as he watches the World Cup on television fortified with that most British of beverages – the cup of tea.

The image, posted on the Iranian president’s Twitter page, is a radical departure from the macabre yet similarly football-themed use of social media by the Islamic State of Iraq and al-Sham (Isis) to show off its gruesome handiwork in Iraq.

The jihadist group, a relentless user of Twitter, posted pictures of atrocities committed in Iraq in recent days – including one resonant image of a group of men, one of whom is wearing a Manchester United shirt, herded together shortly before they are killed.

Mr Rouhani may not have been trying to score an international propaganda coup by having himself pictured watching Iran take on Nigeria wearing a T-shirt and track suit trousers – but he surely has done so.

<noframe>Twitter: Hassan Rouhani - Proud of our boys who secured our first point--hopefully the first of many more to come. <a href="https://twitter.com/search?src=hash&q=%23WorldCup2014" target="_blank">#WorldCup2014</a> <a href="https://twitter.com/search?src=hash&q=%23TeamMelli" target="_blank">#TeamMelli</a> <a href="http://t.co/9pzyT1Dr3f" target="_blank">http://t.co/9pzyT1Dr3f</a></noframe>

15.00 Isis may enjoy backers in Saudi Arabia - and other Gulf kingdoms - but this does not mean the jihadist group enjoys the support of the kingdoms' rulers, says this blog in NOW magazine:

It’s unquestionably the case... that the Gulf monarchs make little if any real effort to prevent their more pious subjects from sending considerable sums of money to extremist outfits, including Isis.

But this is not the same as saying... that Isis is actively and deliberately sponsored as a matter of policy by the Saudis and other Gulf rulers and should be thought of as no more than a proxy created to advance Riyadh’s regional ambitions.

Quite to the contrary: Isis has long been at war with Saudi’s actual clients in Syria...

Isis spokesman Abu Muhammad al-Adnani mocks the Kingdom [Saudi Arabia] as a “state which claims to be Islamic,” and denounces as traitors all factions “supported by the Saudis, America, and the infidels of the West.”

Harishma was set up by a group naming itself the Popular Headquarters for the Defence of Shiite Shrines.

"Those who sign up are organised into units... and if the order is given by the supreme leader [Ayatollah Ali Khamenei] they will go to Iraq to defend the sites," the harimshia.org page said.

13.39 Thousands of Shia Iraqis continue to flee the Isis offensive toward the relative safety of the Kurdish autonomous region in the country's northeast.

The below images are from Aski Kalak, a city midway between Mosul - Iraq's second city, seized by Isis last week - and the major Kurdish city of Erbil.

Displaced Iraqis queue to register at a temporary camp in Aski Kalak (AFP/GETTY)

13.25 Iran's role in the Iraq crisis and measures against Isis has picked up with the arrival in Iraq of the commander of Iran's elite Quds Force, Gen. Ghasem Soleimani.

Interestingly, the US government was notified in advanced of the visit by Soleimani.

Soleimani is no favourite of the Americans, having previously been linked with moves to organise Shia militias targetting US troops in Iraq after the 2003 invasion, as well as more recently providing assistance to Syria's president Bashar Assad.

Ghasem Soleimani (REX)

13.01 Saudi Arabia is "siding with terrorism", according to a strongly worded statement made by the Iraq cabinet.

AFP has this report on unverified claims made by the government of Iraq that Saudi Arabia is responsible for funding received by the Isis jihadists, and should be held responsible for the terrorists' actions:

Saudi Arabia should be held responsible for militant financing and crimes committed by insurgent groups in Iraq, the Baghdad government charged on Tuesday.

Comments from Riyadh indicates it is "siding with terrorism", the cabinet said in a statement issued by Prime Minister Nuri al-Maliki's office.

"We strongly condemn this stance," the statement read.

"We hold it (Saudi Arabia) responsible for what these groups are receiving in terms of financial and moral support."

It continued: "The Saudi government should be held responsible for the dangerous crimes committed by these terrorist groups."

The statement came just days after Saudi Arabia and Qatar blamed "sectarian" policies by Iraq's Shiite-led government against the Sunni Arab minority for the unrest that has swept the country.

The unrest "could not have taken place if it was not for the sectarian and exclusionary policies implemented in Iraq over the past years that threatened its stability and sovereignty," the Saudi government said in a statement.

In March, Maliki accused both Saudi Arabia and Qatar of supporting terrorism in Iraq.

12.19 At leat 44 prisoners being held at a police station were killed in the overnight Isis assault on Baquba, a city close to Baghdad, officials have said.

Accounts varied over whether they had been killed by jihadists carrying out the assault, or security forces trying to prevent them from escaping.

11.43 The Isis jihadists' attack on Baquba (see 09.25) overnight represents the closest that fighting has come to the capital Baghdad so far.

The assault on Baquba - 37 miles northeast of Baghdad - was repelled by security forces.

The jihadists have said they intend to march on Baghdad and the southern Shiite holy city of Karbala.

The swift advance of the militants has sparked international alarm, with UN envoy to Baghdad Nickolay Mladenov warning that Iraq's sovereignty is at stake.

"Right now, it's life-threatening for Iraq but it poses a serious danger to the region," Mladenov told AFP.

"Iraq faces the biggest threat to its sovereignty and territorial integrity" in years, he added.

Iraqi Shiite gunmen carry weapons during a demonstration in Baghdad's Shuala district (EPA)

11.32 The prime minister of Iraq's autonomous Kurdish region, Nechirvan Barzani, has said it is "almost impossible" for Iraq to return to its pre-Isis offensive state.

The various factions - Iraqi Sunni and Shia as well as Kurdish - needed to "sit down and find a way to live together", but the country cannot stay together as it was, hetold the BBC.

Isis has made major territorial gains since seizing Mosul last week, while the Kurds have also occupied key locations not previously part of their autonomous region, including Kirkuk, since the Iraqi army fled in the face of the Sunni jihadists' assault.

11.19 Video has emerged of hundreds of Shia volunteers in Baghdad signing up to join the fight against Islamic militants in Iraq.

The volunteers, who have pledged to join the nation's beleaguered security forces to battle against Sunni militants from the Islamic State of Iraq and the Levant, or Isis, gathered at the Federal Police Command headquarters in the Iraqi capital.

They then set off in a convoy of trucks and buses towards the front line.

10.55 Iraq's biggest oil refinery, Baiji, has been shut down and its foreign staff evacuated, refinery officials said on Tuesday, adding that local staff remain in place and the military is still in control of the facility.

Isis jihadists have advanced into the oil refinery town of Baiji and surrounded the refinery.

Global oil prices have shot up since the Isis offensive in Iraq began, but dropped slightly on Tuesday with the announcement of US troops being deployed in the country.

Isis supporters wave al-Qaida flags in Mosul (AP)

10.26 William Hague has said the "circumstances are right" to reopen Britain's embassy in Iran.

The comments follow talks this weekend between Hague, the British foreign secretary, and his Iranian counterpart over cooperation on the Iraq crisis.

10.06 Fighting in Syria between Isis and the Assad regime appears to have restarted after a two-week hiatus.

Isis was initially welcomed by anti-government rebels in the Syrian civil war, but its abuses and ambitions for an Islamic caliphate straddling Syria and Iraq quickly alienated its erstwhile allies there.

While Isis has been pushed out of the northern Syrian regions around Aleppo, it retains a strong presence in Raqa, Hasakey and Deir Ezzor.

Fighting was reported in Deir Ezzor on Monday night, as well as a car bomb in Shmeytiyeh village, targeting a base belonging to Al-Nusra and Islamist rebel brigades fighting Isis. Six are reported dead.

Map showing Deir Ezzor in relation to northern Iraq

10.00 Iraqi state news channel "Iraqiya" has said that the army killed two Isis commanders on Tuesday. It did not say where in Iraq. More details to follow

09.49 We also have reports of another repelled Isis attack, this time on the northern Iraq village of Basheer, some nine miles south of the Kurdish city of Kirkuk.

Like Tal Afar, which fell to jihadists yesterday, Basheer is predominantly inhabited by Shia ethnic Turkmens.

Isis jihadists were beaten back from Basheer by a mixture of local militia and police forces, after the Iraqi army fled the region some days ago, according to reports.

The US national security council held a late night session to consider options for military action to support Iraq's government hours after US and Iranian officials discussed cooperation on stabilising the region.

President Obama has made U.S. action contingent on Nouri al-Maliki broadening his Shia-dominated government.

"The President will continue to consult with his national security team in the days to come," the White House said, without elaborating.

Among those attending the meeting were Secretary of State John Kerry, Defense Secretary Chuck Hagel, CIA Director John Brennan and Chairman of the Joint Chiefs of Staff Martin Dempsey.

Full diplomatic relations were suspended after attacks on the British embassy in Iran in 2011.

A Iraqi young boy and tribesmen hold up their weapons as they gather to show their readiness to join Iraqi security forces (AFP/GETTY)

09.25 An overnight attack took place in the centre of Baquba, capital of Diyala provinnce, and according to the officers, saw militants temporarily occupying several neighbourhoods.

Militants attacked and took control of parts of the central Iraq city of Baquba but security forces eventually repelled the assault on Tuesday, army and police officers said.

Baqubah is around 30 miles northeast of Baghdad.

09.20 Iraqi government forces still hold parts of the northern town of Tal Afar, in Nineveh province, which was largely seized by Isis jihadists yesterday, according to deputy provincial council chief Nuriddin Qabalan.

08.45 Video footage has been released of the comments made yesterday by US Secretary of State John Kerry, who said America is willing to talk with Iran over ways the two long-time foes might help stop the insurgents known as Isis. In an interview with Yahoo! News, Kerry said Washington is "open to discussions" with Tehran if the Iranians can help end the violence and restore confidence in the Iraqi government.

Today is the second day of critical nuclear talks between Iran and six world powers including the US and Britain, with just five weeks until a July 20 deadline to reach a deal and gaps between the two sides seemingly still huge. Talks on Iraq have reportedly taken place on the sidelines of the conference.

08.25 Cold blooded "executions" said to have been carried out by militants in northern Iraq almost certainly amount to war crimes, the United Nations said on Monday. Richard Spencer reports for the Telegraph from Erbil, Iraq:

After the Islamic State of Iraq and al-Sham (Isis) released graphic photographs of its fighters shooting scores of young men, Navi Pillay, the UN High Commissioner for Refugees, voiced shock over the bloodshed.

Isis claims to have executed 1,700 people after capturing the Iraqi city of Tikrit. Ms Pillay said the figure could not be verified, but added: "This apparently systematic series of cold-blooded executions, mostly conducted in various locations in the Tikrit area, almost certainly amounts to war crimes."

Navi Pillay (REUTERS)

08.00 About 100 additional US forces are being put on standby, most likely in Kuwait, and could be used for airfield management, security and logistics support, officials said.

Separately, three U.S. officials said the White House was considering sending a contingent of special forces soldiers to Iraq. Their limited mission - which has not yet been approved - would focus on training and advising beleaguered Iraqi troops, many of whom have fled their posts across the nation's north and west as the al-Qaida-inspired insurgency has advanced in the worst threat to the country since American troops left in 2011.

07.47 The survival of Nouri al-Maliki's democratically elected government is of crucial importance to the West, our leader argues this morning:

Mr al-Maliki, whose failure to address Iraq’s long-standing sectarian divisions has helped create the current crisis, clearly does not enjoy a high level of support in London and Washington. But that is no justification for allowing events to take their bloody course in the hope that Mr al-Maliki will prevail. Where Britain’s national interests are concerned, making sure Iraq’s democratically elected government survives the current crisis is far preferable to the barbarous, Islamist alternative.